So-called high performance water reducing agents have been known for use in concrete admixtures and can impart high fluidity. Examples thereof include salts of naphthalenesulfonic acid/formaldehyde condensates (naphthalene derivatives), salts of melaminesulfonic acid/formaldehyde condensates (melamine derivatives), salts of sulfanilic acid/phenol formaldehyde co-condensates (aminosulfonic acid derivatives), salts of polycarboxylic acids (polycarboxylic acid derivatives) and so on.
Each of these admixtures has some problems even though each has excellent functions. For example, the naphthalene derivatives and the melamine derivatives are excellent in hardening characteristics, however, are problematic with regard to retention of fluidity (owing to their tendency to cause a slump loss). On the other hand the polycarboxylic acid derivatives is disadvantaged due to its retarding effect on the hardening property of the composition. Recently polycarboxylic acid concrete admixtures which can impart excellent fluidity have been developed, which makes it possible to attain excellent dispersion with a reduced amount of an admixture, and thereby the problem of retarding the hardening property is being solved. Such admixtures include water-soluble vinyl copolymers such as copolymers of polyalkylene glycol monoester monomers having unsaturated bonds with acrylic acid and/or unsaturated dicarboxylic acid monomers (see JP-A 58-74552, JP-A 62-78137, JP-A 62-119147, JP-A 3-75252 and JP-A 59-162160).
These water-soluble copolymers each exhibit an excellent dispersing effect even at relatively low concentrations, so that the retardation is improved comparatively. Further, as described in JP-A 59-162160, they are somewhat effective in maintaining the dispersion. However, the copolymers cannot satisfactorily effect on retention of fluidity for a long time.
Under these circumstances, the inventors of the present invention have proposed in JP-A 7-223852 (corresponding to WO-A 95/16643) an improvement of slump loss by the use of a copolymer having a long polyoxyalkylene chain as an admixture. The technique described therein makes it possible to maintain the slump at a suitable level for a long period of time for example 60 to 90 minutes.
However, ideally the slump is maintained for about 2 hours to cope with inevitable delays in concrete processing due to traffic jams or troubles in construction. Therefore, the above effect of maintaining the slump for about 90 minutes is still insufficient and ideally would be further improved. Further, an improvement in the effect of maintaining the slump is liable to cause a problem of retarding the hardening property, so that such an improvement is also required to be attained without retarding the hardening property.